Dysphagia is a common complication with diseases such as stroke, neurodegenerative diseases, brain tumors, respiratory disorders, and the like wherein insufficient control of muscles needed for swallowing engender a risk of aspiration pneumonia. Aspiration pneumonia has been estimated to inflict a 20% death rate in the first year after a stroke and 10-15% each year thereafter. Treatment for this disorder requires either feeding through a nasogastric tube on a temporary basis or enteric feeding through a stoma to the stomach in chronically affected cases. The treatment costs and the commensurate value of a remediation technology if one were available, are very high. For example, in 1992 Medicare paid for enteral feedings of 206,000 patients at a cost of $505 million in one year. Furthermore, this cost is an underestimate because Medicare pays only half of the enteric home feeding costs and approximately 412,000 patients per year receive enteric feedings due to risk of aspiration in the United States alone. Accordingly, any technology that can significantly reduce the number of patients who require this extra care due to insufficient control of muscles used for swallowing would provide great monetary and quality of life benefits to the nation.
Dysphagia often results from poor control of some muscles in the upper respiratory system. Many muscles in this system affect important complex movements during speech and voice. Patients sometimes lack proper control of muscles used for these other activities and unfortunately remedial efforts leave much to be desired here as well. Electrical stimulation of upper respiratory system muscles has been used to alleviate pain and to stimulate nerves, as well as to treat disorders of the spinal cord or peripheral nervous system. Stimulation further has been used to facilitate upper respiratory muscle reeducation and in conjunction with other physical therapy treatments.
Generally the technique of stimulating muscles in the body has been used to induce contraction of individual muscles in other systems. For example, stimulator implants have been used to modulate and synchronize bladder and sphincter function via two different alternately stimulated muscles, as described in U.S. Pat. No. 6,393,323 issued to Sawan et al., on May 21, 2002. In some cases, stimulator implants may amplify volitional control of a specific muscle by electrode detection of early muscle contraction followed by a stimulatory pulse sent to the electrode as described in U.S. Pat. No. 6,354,991 issued to Gross et al., on Mar. 12, 2002. One embellishment to this technique is biphasic stimulation with a first anodal sub-threshold stimulation followed later in time by a cathodal stimulation for the same muscle as described in U.S. Pat. No. 6,343,232 issued to Mower et al., on Jan. 29, 2002. Still further improvements include, for example, the use of electrodes that remain at a desired implantation site and that accommodate expansion of muscle during muscle flex, such as Peterson-like electrodes and flexible electrode leads.
These advances are helpful but generally do not address sufficiently the control of specific cartilage, tissue or bone movements, which require the coordinated action of multiple muscles. For example, at least 12 muscles are involved in moving the hyoid bone. Proper control of this movement is particularly important due to the consequences from failure of movement of this bone to raise the larynx to protect the airway and open the upper esophageal sphincter to clear liquid or food from the hypopharynx. That is, normal swallowing involves hyolaryngeal muscle contractions that synchronize with and control the opening of the upper esophageal sphincter. The apparently intricate orchestration of muscle movements needed for this double action has not been previously controlled by stimulation of hyoid associated muscles through implanted electrodes.
Some attempts to control upper respiratory muscles used for swallowing have targeted the hyoid associated muscles through exterior skin electrical contact. For example, Freed et al. have described a non-invasive method and apparatus that continuously stimulates the skin surface to assist patients in initiating a swallow (U.S. Pat. Nos. 5,725,564; 6,104,958 and 5,891,185). The Freed et al. device is a temporary basis sensory stimulation tool for early rehabilitation of stroke patients that have difficulty initiating swallowing behavior. This device may have some value for swallowing rehabilitation therapy. However, no suitable description of a chronically implanted (i.e. implanted for multiple stimulations) neuroprosthestic system exists for long term prevention of aspiration during swallowing in patients who have not been able to take food or liquids by mouth following unsuccessful rehabilitation. The Freed et. al. rehabilitation device is not appropriate for patients with a chronic disorder that require enteric feeding due to the risk of aspirating food.
Another problem with the Freed et al., technology is the inability to produce direct movement or muscle contraction. More specifically, the Freed device does not demonstratively elevate the larynx, move the hyoid bone or open the upper esophageal sphincter. It appears that this device and the method of its use operate by creating a sensory input without directly causing any muscle contraction or other action involving the larynx. This research group commented on the latter limitation to their method, stating “[m]uch research is required to determine whether ES (electrical stimulation), applied at a sensory level in our study, works via a peripheral nerve, a direct effect on the small muscles, the central nervous system, or a combination of these factors.” (Freed et al. Respiratory Care, 46: 466-474, 2001). Accordingly, although the Freed group seems to have made some progress using an externally applied electric current, a major conclusion from their limited success is that a suitable route for direct control of the muscles involved in swallowing remains unknown.
Despite the hints that basic research is needed in this area, dysphagia conceivably might be alleviated by direct control of muscles that are no longer receiving the correct signals from the brain. However, the route for alleviating dysphagia by direct control of muscles has not been tried with convincing success. Although Bidus et al. showed that stimulation of the thyroarytenoid vocal fold muscles in the larynx with percutaneously inserted hooked wire electrodes could close the glottis and improve the voice in patients with abductor spasmodic dysphonia (Bidus et al. Laryngoscope, 110:1943-1949, 2000), no synergistic production of laryngeal elevation and opening of the upper esophageal sphincter were attempted.
Another group found that chronic stimulation of canine thyroarytenoid vocal fold muscles with Peterson-like type electrodes could close the glottis intermittently during 6 months of chronic implantation in the canine (Ludlow et al. Journal of Artificial Organs, 23:463-465, 1999; and Ludlow et al. Muscle and Nerve, 23:44-57, 2000). However, the studies did not address elevation of the larynx or opening of the esophageal sphincter. More pertinently, individual laryngeal muscle stimulation in humans has been explored but synergistic anterior movement of the hyoid bone with simultaneous opening of the upper esophageal sphincter, as needed to prevent aspiration, were not examined. Furthermore, although at least twelve muscles are known to have involvement in swallowing, there has been no clear understanding of which muscles may predominate or even if proper swallowing requires coordinated contraction of all twelve or more muscles. In addition, the system may be complicated in unexpected ways by individual differences. For example, the geniohyoid, mylohyoid and digastric muscles are used selectively by different individuals, with some using all three muscles at the onset of swallowing, and others using different pairs (Spiro et al., Laryngoscope 104: 1376-82 1994). In addition, the temporal association between submental muscle contractions differs across individuals (Hrycyshyn et al., Am. J. Anat. 133: 333-40 1972). Thus, despite work in this area, muscular control by imbedded electrode(s) to coordinately control a solid internal body part such as a cartilage, tissue or bone through two or more muscles and thereby emulate normal synergistic movement has not been possible.